Cartons having so-called "gable-tops", "slant-tops", or "fold-flat-tops" are widely used as disposable containers for liquid products, such as milk. These cartons are generally made from paperboard which is coated, on both sides, with a plastics material (such as polyethylene). This material forms protective layers which also serve as a heat activated adhesive. In making the carton, a coated paperboard carton blank is formed into a sleeve having a square or rectangular cross-section. One pair of opposite panels at the top of the sleeve are subsequently formed into the sloping or outer walls of e.g. the gable top. (These sloping walls are similarly present in, e.g. a "fold flat-top carton" during the initial stages of making the carton, since the top is subsequently folded flat.) The other pair of opposite walls are folded inwardly, in bellows fashion, and are tucked under the sloping walls before the marginal portions of the panels are heat sealed together so as to form a ridge-shaped seal at the top of the carton. (The ridge-shaped seal is formed on the slant in a "fold flat-top" carton so the top can be subsequently folded flat.) One of the panels which is tucked under the sloping or outer walls will be referred to as the pouring spout panel and this panel is revealed when the triangular wings are folded back to enable this panel to be drawn forwardly to form the pouring spout.
Such cartons are normally transported from a filling station (where each carton is filled with liquid) to a heating station (where the marginal portions of the panels are heated to activate the adhesive) and then to a sealing station (where the marginal portions are pressed together to form the ridge-shaped seal). The heating station usually includes an "oven" for heating the marginal portions of the carton top by streams of hot air and the sealing station usually includes parallel jaws for pressing the heated marginal portions together in order to form the ridge-shaped seal.
As these cartons are of a comparatively simple and inexpensive construction, i.e. for use as disposable containers, problems are encountered in forming hermetic seals and also in making sealed cartons which are easy to open. Whilst various attempts have been made to form easy-to-open hermetic seals there is a conflict between making a reliable hermetic seal and weakening the seal to facilitate opening.
A conventional technique for making cartons easier to open employs an "abhesive" coating on certain zones of the marginal portions of the carton. The abhesive coating reduces adhesion and thereby enables the pouring panel to be easily released when the carton is opened. However, the use of abhesive has certain disadvantages. Besides the cost of the abhesive, there is the cost involved in modifying carton manufacturing machines to coat abhesive onto selected zones of the marginal portions of the carton, and the cost of maintaining such a machine under working conditions. Also, for reasons which will be explained below, abhesive coatings can be rendered ineffective by overheating and/or by splashing when operating at high filling speeds.
An attempt to solve the problem of making an easy-to-open hermetic seal is disclosed in WO-A-90/09926. One of the essential characteristic features of this prior art solution is to leave an area (which may be in the shape of an inverted triangle, or a notch, or sector) unsealed, or lightly sealed at the top centre of an "end flap" which forms part of the pouring spout. This "unsealed" area is intended to assist in facilitating the step of drawing the pouring panel forwards, after folding back the triangular wings, since an area at the top central position is left unsealed. This solution also mentions the use of a stake point located at the apex of the triangle to provide an effective seal. Use can also be made of an abhesive at the unsealed area to prevent adhesion. This solution apparently employs a particular post-pasteurisation process in combination with the weakened seal.
Despite such prior art attempts to solve the problem of an easy-to-open hermetic seal, various problems can generally arise in making effective ridge-shaped hermetic seals when operating at higher filling speeds. Filling speeds are limited by certain physical requirements but the number of cartons which can be filled per minute depends, to some extent, on the volumetric capacity of the cartons. For example, a relatively slow speed is used to fill larger cartons having, e.g. capacities of two pints and one liter. However, a relatively faster speed cam be used to fill smaller cartons with capacities of, e.g. 1/3, 1/2 and 1 pint, and 1/2 liter. For obvious economic reasons, filling plants wish to fill cartons as fast as possible. However, if the filling speed is too high, the liquid expelled from filling spouts can splash onto regions of the carton which are coated with the plastics material that is activated (by heat) to form the adhesive bond in the ridge-shaped seal. It can similarly splash onto zones of the carton which may be coated with abhesive. In addition to this, each carton will spend less time at the heat sealing station, due to the higher throughput, and then more heat needs to be applied to activate the heat sensitive plastics coating in a shorter space of time. If the amount of heat is not increased in an attempt to create an easy-to-open seal, the bond between the marginal portions may not be strong enough (in certain areas) and this can lead to leakage when the sealed cartons are subsequently handled and transported to consumer outlets. If more heat is applied, e.g. by increasing the size of the zones heated by air streams, this will increase the strength of the seal in places where a weaker bond is required to facilitate opening of the carton. In this case, although an hermetic seal can be made, the cartons are difficult to open. If more heat is applied by increasing the temperature and/or volume of flow of the air streams the plastics coating can then be overheated to an extent which adversely affects the ridge-shaped seal. Moreover, where abhesive coatings are used, the abhesive can be rendered ineffective by such overheating. Also, whether or not abhesives are used, streaks of adhesive can run over zones which are intended to be left unsealed, or caused to be lightly sealed, and this subsequently increases the strength of the seal in places where a weaker bond is required to facilitate opening.
Problems can also be caused by stretching of chains which are in used in the transporting systems that convey cartons between the filling station and the heating and sealing stations. If the chains stretch, each carton can be misaligned with respect to the "oven" in the heating station and the heated air streams will be misdirected towards zones on the marginal portions of the panels which form the ridge-shaped seal. Similar misalignment can occur at the sealing station and the heated marginal portions will not then be pressed together properly, between jaws, to form the ridge-shaped seal.
A particular difficulty is experienced in sealing a central region of the ridge-shaped seal, i.e. where the bellows-shaped panels meet, but leave a small gap, between confronting V-shaped folds. Liquid can escape through this gap, particularly where efforts are made to weaken the ridge-shaped seal so as to facilitate release of the pouring spout.
Carton leakage may not be present or easily detectable when the cartons are filled and sealed and leakage is far more likely to occur when the cartons are packed and transported to consumer outlets. For example, the cartons may be packed in tens or twelves in shrink-wrappings which are stacked five cartons high. In view of the internal pressures caused by stacking and handling, a carton may start to leak and this will be visibly detected by liquid gathering in the bottom of the shrink wrapper. The whole pack of cartons in such a wrapper will be subsequently rejected by the retail outlet or end user.
As the prevention of leakage is far more important than solving the problem of making the carton easy to open, the tendency is to reduce the risk of leakage by using more heat over larger zones of the marginal portions of the carton so as to ensure that a good hermetic seal is made. However, as this is at the expense of facilitating opening of the carton, cartons sealed in this way are undesirable end products to the consumer.